With the advance of weight-, thickness- and length-reducing technology in the field of the electronic and electric industry and with the recent advancement of weight-reducing technology in the fields of the automobile, aircraft and space industries, there has been a strong demand for crystalline thermoplastic resins having heat resistance of about 300.degree. C. or higher and permitting easy melt processing in recent years.
As crystalline, heat-resistant, thermoplastic resins developed to date, there are, for example, poly(butylene terephthalate), polyacetal, poly(p-phenylene thioether) (PPS), etc. These resins are however unable to meet the recent requirement level for heat resistance.
Polyether ether ketones (PEEKs) and polyether ketones (PEKs) have recently been developed as heat-resistant resins having a melting point of about 300.degree. C. or higher. These resins are crystalline thermoplastic resins. It has therefore been known that conventional melt processing techniques such as extrusion, injection molding and melt spinning can be applied to easily form them into various molded or formed articles such as extruded products, injection-molded products, fibers and films. These resins however use expensive fluorine-substituted aromatic compounds such as 4,4'-difluorobenzophenone as their raw materials. Limitations are thus said to exist to the reduction of their costs. It is also pointed out that these resins involve a problem in expanding their consumption.
Based on an assumption that PTKs could be promising candidates for heat-resistant thermoplastic resins like PEEKs and PEKs owing to their similarity in chemical structure, PTKs have been studied to some extent to date. There are some disclosure on PTKs, for example, in Japanese Patent Laid-Open No. 58435/1985 (hereinafter abbreviated as "Publication A"), German Offenlegungsschrift 34 05 523A1 (hereinafter abbreviated as "Publication B"), Japanese Patent Laid-Open No. 104126/1985 (hereinafter abbreviated as "Publication C"), Japanese Patent Laid-Open No. 13347/1972 (hereinafter abbreviated as "Publication D"), Indian J. Chem., 21A, 501-502 (May, 1982) (hereinafter abbreviated as "Publication E"), and Japanese Patent Laid-Open No. 221229/1986 (hereinafter abbreviated as "Publication F").
Regarding the PTKs described in the above publications, neither molding nor forming has however succeeded to date in accordance with conventional melt processing techniques. Incidentally, the term "conventional melt processing techniques" as used herein means usual melt processing techniques for thermoplastic resins, such as extrusion, injection molding and melt spinning.
The unsuccessful molding or forming of PTKs by conventional melt processing techniques is believed to be attributed to the poor melt stability of the prior art PTKs, which tended to lose their crystallinity or to undergo crosslinking and/or carbonization, resulting in a rapid increase in melt viscosity, upon their melt processing.
It was attempted to produce some molded or formed products in Publications A and B. Since the PTKs had poor melt stability, certain specified types of molded or formed products were only obtained by a special molding or forming process, where PTKs were used only as a sort of binder, being impregnated into a great deal of reinforcing fibers of main structural materials and molded or formed under pressure.
Since the conventional PTKs are all insufficient in melt stability as described above, it has been unable to obtain formed products such as films by applying conventional melt processing techniques.